Metal oxides have been of great importance to the development of energy conversion and storage technologies including heterojunction solar cells, Li-ion batteries, and electrocatalysts/photocatalysts for water splitting and CO2 reduction. The role of metal oxides in these devices has been diverse, from charge transport layers to catalytic surfaces to protective blocking layers. Understanding the fundamental structural and electronic properties of these materials will continue to allow for advancement in the field of renewable energy. Electrochemical impedance spectroscopy (EIS) is one of the most utilized methods to characterize these electrodes in the context of energy applications. The utility of EIS stems from its ability to differentiate multiple interfaces (i.e., solid/electrolyte, solid/solid) within devices on the basis of their frequency response to a modulated potential and the subsequent decoupling of resistive and capacitive circuit components. In this review, the fundamental theory of EIS is first described with a physical and mathematical basis, followed by a discussion of equivalent circuit modeling. The review then covers examples from the literature where EIS has been particularly important in the understanding of electronic properties related to metal oxide electrodes within energy conversion and storage devices. A specific focus is placed on metal oxides that are used as heterojunction solar cells, ion batteries, and photocatalysts/electrocatalysts. Common themes are discussed within each application such as the study of electron and hole diffusion in solar cells, the dependence of recombination reactions and catalysis on surface defect/trap states for solar cells and photocatalysts, and the formation of passivation layers at the solid electrolyte interface in Li-ion batteries.

Delafossite CuGaO2 nanocrystals were hydrothermally synthesized and characterized spectroscopically and electrochemically as mesoporous thin films. The nanocrystals demonstrate a preferred orientation within the film structure, as shown by enhancement of the (00l) peaks via two-dimensional powder X-ray diffraction. Annealing conditions of low and high temperature (i.e., 100–300 °C), with oxygen and/or argon atmospheres, were investigated, and the resulting effect on the thin film electrochemistry was measured. Cyclic voltammetry showed an increase in non-faradaic current with higher annealing temperatures and demonstrated a quasi-reversible redox feature (E 1/2 = 0.1 V vs Fc+1/0). This feature is assigned to a CuII/CuI redox couple associated with surface defects. X-ray photoelectron and energy dispersive spectroscopies provide evidence for CuII surface defects and copper vacancies. Electrochemical impedance spectroscopy revealed that CuGaO2 films were highly conductive with σ ∼ 10–5 Ω–1 cm–1, consistent with a large density of hole carriers induced by copper vacancies. The significance of synthesis, film preparation, and annealing conditions on the presence of surface defects and large hole densities is discussed. The prevalence of such defects in delafossite CuGaO2 is expected to have a large impact on the use of this material as a hole transport layer in solar cell architectures.

Nanocrystalline MnFe2O4 has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been sparingly studied as highly ordered thin-film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe2O4 and Fe3O4 spinel oxide films were grown on (001)- and (111)-oriented Nb:SrTiO3 perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). High-resolution X-ray diffraction (HRXRD) and X-ray photoelectron spectroscopy (XPS) show the synthesis of pure phase materials, while scanning transmission electron microscopy (STEM) and reflection high-energy electron diffraction (RHEED) analysis demonstrate island-like growth of (111) surface-terminated pyramids on both (001)- and (111)-oriented substrates, consistent with the literature and attributed to the lattice mismatch between the spinel films and the perovskite substrate. Cyclic voltammograms under a N2 atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe2O4 and Fe3O4. Under an O2 atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, a diffusion-limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe2O4 reported in the literature where the diffusion-limited current is achieved with Mn-based catalysis. This difference is attributed to a low density of Mn surface termination, as determined by the integration of current from CVs collected under N2, in addition to low conductivity through the MnFe2O4 film due to the degree of inversion. Such low densities are attributed to the synthetic method and island-like growth pattern and highlight challenges in studying ORR catalysis with single-crystal spinel materials.

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A solid-state quantum dot solar cell (QDSC) is implemented with cadmium sulfide (CdS) quantum dots (QDs) tethered to titanium oxide (TiO 2 ) as photoanode, carbon fabric (C-fabric) as counter electrode (CE), and a solid electrolyte of succinonitrile/Na 2 S mixed in a 2:1 molar ratio (SN/S 2− ) as the hole transport material. Electron transfer and recombination processes are investigated by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) for a TiO 2 /CdS/ZnS-SN/S 2− -C-fabric (solid state) and TiO 2 /CdS/ZnS-S 2− -C-fabric (liquid junction) devices at different white light intensities. IMPS and IMVS studies show that the electron transport rate and electron recombination decrease with an increase in the intensity of light. The champion cell with a TiO 2 /CdS/ZnS photoanode and C-fabric as CE delivers a power conversion efficiency (PCE) of 5.2% with a solid electrolyte of SN/S 2− , and a PCE of 6.3% is obtained with the liquid aqueous electrolyte of 0.1 M Na 2 S. The effects of temperature on solar cell performance and the thermal dependence of ionic conductivity for the solid electrolyte are studied in detail. This study shows that the succinonitrile based solid electrolyte is a good substitute for the traditional liquid electrolyte.